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In Case You Missed It:

Salt and your health, Part I: The sodium connection

Published: October, 2010

By now, most health-conscious American men understand the
difference between bad fats (saturated fats and trans fats) and
good fats (omega-3s and other poly- and monounsaturates). Many
also recognize the difference between bad carbohydrates (simple
sugars and refined grain products) and good carbs (dietary fiber
and whole-grain products). In addition, savvy consumers are
finally switching from red meat to fish, poultry, and legumes to
get the protein they need.

It's heartening progress, but it overlooks another nutrient
that's responsible for more than 100,000 American deaths a year,
about three times more than prostate cancer. Perhaps because the
problem nutrient is hidden away in processed foods and receives
massive support from companies that manufacture and market these
foods, the average American consumes 55% more of it today than in
1980. The hidden nutrient is not a fat or carbohydrate, and it
doesn't pack any calories. The forgotten nutrient is salt.

A grain of history

In today's world, salt is abundant and cheap, but it wasn't
always that way. Salt was hard to come by for our earliest
ancestors, who got along quite nicely on about a tenth of today's
average use in the United States. In time, people learned how to
find salt and extract it from the earth. But it was hard work and
salt was scarce, so it became a valuable commodity that was used
for currency. In fact, the word salary is derived from
the Latin word for salt. Perhaps because it was rare and
expensive, salt carried a certain prestige; even today, a
successful man is "worth his salt" and a good man is "the salt of
the earth."

After the Industrial Revolution, salt became inexpensive and
plentiful. It found a valuable role as a food preservative, and
the average consumption soared to as much as 7,000 milligrams
(mg) a day in the 19th century. Salt has long since outlived its
use as a preservative, but our hankering for sodium lingers on,
with daily consumption in America averaging 3,436 mg. Because of
this acquired preference, salt is a big business: every year, the
world consumes about 187 million tons, which is both mined from
the earth and claimed from the sea.

Salt and sodium: A glossary

Each molecule of ordinary salt is composed of an atom of
sodium (Na) joined to an atom of chloride (Cl); the
chemical designation is NaCl. Because chloride is heavier
than sodium, it contributes more to the weight of the
molecule. But when it comes to health, it's the sodium
that counts, whether it comes from table salt or from
other sources, such as baking soda (sodium bicarbonate)
or MSG (monosodium glutamate).

Because sodium is what matters, food labels list the
content of sodium, not salt; it's expressed as milligrams
(mg) of sodium. Most current dietary guidelines also
specify milligrams of sodium, and it's the designation
used by this and many other publications. But some
nutritional information is still expressed in milligrams
(mg) or grams (g) of salt. And to make things even more
confusing, many research papers use another unit,
millimoles (mmol).

Milligrams will do nicely for most of us; it's complex
enough, especially if you're not used to the metric
system. But if you encounter the other terminology, you
can make your own conversions using these round numbers:

1,000 mg sodium = 1 g sodium

1 g sodium = 2.5 g salt

1 mmol sodium = 23 mg sodium

It is confusing, but it shouldn't shake your
determination to keep track of the sodium in your diet.

It's the sodium, stupid

For chemists, a salt is any molecule that forms when positively
and negatively charged atoms bond with each other. But when the
atoms are sodium and chloride, the compound takes the name
salt all to itself. For physicians, sodium is the key
element in salt. It's a crucial as well as controversial
substance; perhaps that's why its name evolved from the Arabic
suda, "a splitting headache."

A taste of physiology

Make no mistake about it: salt is essential for human health. The
average adult's body contains 250 grams (g) of sodium — less than
9 ounces, or about the amount in three or four saltshakers.
Distributed throughout the body, salt is especially plentiful in
body fluids ranging from blood, sweat, and tears to semen and
urine.

Sodium is absorbed from the gastrointestinal tract, always
bringing water along with it. It is the major mineral in plasma,
the fluid component of blood, and in the fluids that bathe the
body's cells. Without enough sodium, all these fluids would lose
their water, causing dehydration, low blood pressure, and death.

Fortunately, it only takes a tiny amount of sodium to prevent
this doomsday scenario; in fact, some isolated population groups
manage perfectly well on just 200 mg a day. About one-quarter of
the tongue's taste buds are devoted to recognizing salt; like
other animals, humans can — and do — seek out salt when they need
it. And when dietary salt is in short supply, the body can
conserve nearly all its sodium, dramatically reducing the amount
excreted in urine and shed in sweat. Remember that water always
follows sodium, and you'll understand why your skin is dry and
your urine scant and concentrated when you are dehydrated and
conserving sodium.

To be sure its supply of salt and water is just right, the body
has developed an elaborate series of controls. The blood vessels
and brain signal the kidneys to retain or excrete sodium as
needed; they also fine-tune the sensation of thirst so you'll
provide water in amounts that match the body's sodium supply.

The body, in its wisdom, can make do with remarkably small
amounts of sodium. But human behavior can thwart nature's checks
and balances by taking in much more sodium than we need. The
major consequence is a rise in blood pressure, which leads to a
heightened risk of heart attack and stroke.

Other benefits

Reducing dietary salt will lower blood pressure,
protecting against heart attack and stroke. That's reason
enough to shake the salt habit, but there's more. Even
modest salt restriction improves vascular reactivity and
reduces urinary albumin loss, which protects the kidneys
and the heart. Salt restriction also lowers the risk of
kidney stones by reducing the amount of calcium in the
urine. And the DASH diet appears to protect against
diabetes, at least in Caucasians.

Sodium and blood pressure

Scientists know that sodium has an important influence on blood
pressure, but they are not sure exactly how it works. It's no
surprise, since the systems that control blood pressure include
dozens of complex vascular, neurological, and hormonal elements.
Although the body can rid itself of excessive dietary sodium, it
seems likely that eating salt expands your blood volume, at least
to a subtle degree. In turn, the extra volume may signal your
kidneys to trigger a cascade of hormonal and vascular effects
that raise blood pressure. And some experts suspect that these
hormones may have adverse effects on vascular health even if
blood pressure remains stable. In fact, a 2009 Australian study
reported that a low-sodium diet improves arterial function
independent of any effect on blood pressure.

Sodium and hypertension

The first person to suspect that eating salt might contribute to
high blood pressure may have been Emperor Huangdi of China; about
5,000 years ago he wrote: "If too much salt is used in food, the
pulse hardens." The scientific discourse, however, dates only
from 1972, when Dr. Lewis Dahl presented evidence that a diet
high in sodium contributes to high blood pressure. His hypothesis
was soon questioned by other researchers, and the sodium
controversy has raged ever since.

Why did the link between sodium and blood pressure generate so
much heat? Part of the reason stems from the body's intrinsic
complexity: sodium is but one of an enormous number of factors
that affect blood pressure — and for all its importance, blood
pressure is only one of the many things that determine vascular
health. And the complexities of human behavior are just as
daunting as those of human biology; dietary potassium, calcium,
and many other nutrients influence blood pressure, as do
exercise, body weight, alcohol use, and stress.

Additional challenges result from the methods scientists use to
study the link between diet and hypertension. Blood pressure can
fluctuate widely from minute to minute; if sustained, even small
changes in blood pressure can have a large impact on lifetime
risk. Plus, sodium consumption can vary substantially from day to
day. Studies that rely on dietary history can differ from those
that measure the amount of sodium in a person's daily urine,
which should be a more accurate reflection of how much sodium has
been consumed on a given day. Some people are more sensitive to
sodium than others. And experiments that subject volunteers to a
high or low consumption of sodium are necessarily brief, at least
compared to the months and years it takes for blood pressure to
affect health.

Little by little, though, a consensus has emerged. Most
researchers, scientific advisory boards, and government agencies
agree that reducing dietary salt will lower blood pressure,
reduce the risk of heart attack and stroke, and save lives — up
to 150,000 lives a year in the United States alone, according to
the American Medical Association Council on Science and Public
Health. And in this era of rapidly rising health care costs, it's
important to note that cutting down on salt would save us up to
$24 billion a year.

Salt and resistant hypertension

Many excellent antihypertensive drugs are available.
Still, some 10% to 30% of patients fail to achieve good
blood pressure control even while they're taking three
such medications. But that doesn't mean these patients
with resistant hypertension are beyond help. An important
2009 study of resistant hypertension reported that a
low-sodium diet reduced systolic blood pressure by a
whopping 22.7 millimeters of mercury (mm Hg) and
diastolic blood pressure by 9.1 mm Hg.

Sodium restriction will never replace blood pressure
medications — but it sure will help.

Impressive evidence

Although not all studies agree, a large body of evidence points
to sodium as an important contributor to high blood pressure.
After reviewing the results of animal experiments, population
studies, and clinical trials, the World Health Organization
described the evidence that high dietary sodium causes
hypertension as "conclusive." Instead of wading through all the
studies, pro and con, let's focus on just three landmark
investigations.

Epidemiologic evidence

The International Study of Salt and Blood Pressure (INTERSALT)
compared sodium intake, as measured by urinary levels, with blood
pressure in 10,079 people between the ages of 20 and 59 in 52
population samples around the world. To check other factors that
affect blood pressure, each subject was also evaluated for
obesity, alcohol use, and dietary potassium. The result
demonstrated a clear link between dietary sodium and blood
pressure: in communities where the average sodium consumption was
low, only 1.7% of the subjects had high blood pressure, but in
places where sodium consumption was high, 13.4% were
hypertensive.

It didn't take long for scientists to spot a weak link in the
chain between sodium and blood pressure: although the
relationship was clear when one society was compared with
another, there was little if any correlation between dietary
sodium and blood pressure within any one community. That means
Americans who eat a lot of salt don't necessarily have higher
blood pressure than those who eat less. It's a legitimate
criticism, but INTERSALT responded by showing that age is an
important element in the equation. Even within a single country,
such as the United States, blood pressure rises more steeply with
age in people who take in large amounts of sodium than in people
who eat less salt.

This means you

It's easy to dismiss salt as the other guy's problem.
That may be okay if you're a lean twenty-something with a
blood pressure of 110/60, but even these men are likely
to face issues with salt as they grow older.

Current guidelines say no adult should consume more than
2,300 mg of sodium a day, and that people with
hypertension, all middle-aged and older adults, and all
African Americans should consume no more than 1,500 mg a
day.

Where do you fit in? At present, about two-thirds of all
American adults have hypertension or prehypertension, and
the average 50-year-old has a 90% chance of developing
hypertension as he ages. So if you're like the rest of
us, you'll benefit from cutting your dietary salt.

DASHing doubts

Demonstrating a link between dietary sodium and blood pressure is
one thing, showing that cutting down on salt will lower blood
pressure is another. Early trials of reduced sodium diets
produced mixed results, largely because the patient populations,
test diets, and experimental designs varied so greatly. That led
many people to take advice about dietary sodium with a grain of
salt. That skepticism was understandable, at least until 1997,
when a major trial put things in perspective.

The first conclusive evidence that diet can lower blood pressure
came from the Dietary Approaches to Stop Hypertension (DASH)
study. Researchers evaluated three diets: a typical American, or
"control," diet that was low in fruits, vegetables, and dairy
products, with a fat content typical of the American average
diet; a test diet rich in fruits and vegetables; and a
"combination" diet (now known as the DASH diet) rich in fruits,
vegetables, and low-fat dairy products.

Both of the test diets lowered blood pressure, but the DASH diet
was the clear winner, reducing systolic and diastolic blood
pressure by 5.5 and 3 millimeters of mercury (mm Hg),
respectively. People with high blood pressure benefited even
more, reducing their systolic blood pressure by 11.4 mm Hg and
their diastolic pressure by 5.5 mm Hg.

The DASH diet is high in potassium (4,700 mg a day) and dietary
fiber (31 g a day), moderate in calcium (1,240 mg a day), and
moderately low in fat (27% of the total calories). All three
diets contained a similar number of calories, so weight loss did
not account for the benefit of either therapeutic diet. But did
sodium explain the blood pressure improvements?

Surprisingly, perhaps, the answer is no; the study was designed
to provide the same amount of sodium — 3,000 mg a day — in each
diet. So while the original DASH study provided important
evidence that a good diet can lower blood pressure, it offered no
evidence about sodium one way or the other.

Critics of the salt hypothesis argued that DASH provided evidence
against the importance of sodium. It's not true: the study showed
that a good diet can lower blood pressure even if it contains
more sodium (3,000 mg a day) than is currently recommended (1,500
to 2,300 mg a day), but it doesn't address whether additional
sodium restriction can provide additional benefits. That's the
question scientists looked at in the second DASH study.

Like the original research, the second DASH trial compared a
control (typical American) diet with the combination DASH diet.
But researchers also compared the effects of high-,
intermediate-, and low-sodium intakes within each dietary group.

As before, the DASH diet produced a lower blood pressure than the
typical American diet. But in both groups, sodium restriction
produced additional benefits (see box below). Virtually everyone
benefited from sodium restriction, including people with
hypertension and those with normal blood pressures, African
Americans and whites, men and women. The best results were
observed in people with high blood pressure who followed the DASH
diet and also reduced their consumption of sodium to the lowest
levels.

DASHing hypertension

Blood pressure drop when various amounts of sodium are
incorporated in a typical American diet or DASH diet. The
blood pressure in a typical American diet containing
3,400 mg sodium/day is used for comparison.

Diet (sodium/day)

Systolic

Diastolic

Typical (2,300 mg)

2.1 mm Hg

1.1 mm Hg

Typical (1,150 mg)

6.7 mm Hg

3.5 mm Hg

DASH (3,450 mg)

5.9 mm Hg

2.9 mm Hg

DASH (2,300 mg)

7.2 mm Hg

3.5 mm Hg

DASH (1,150 mg)

9.9 mm Hg

4.5 mm hg

Source: Archives of Internal Medicine 2007, Vol.
167, p. 1,463.

Studies confirm that adherence to the DASH diet is associated
with substantial protection against coronary artery disease (24%
reduction in risk), strokes (18% reduction), and heart failure
(37% reduction). These results should be enough to convince you
to shake the salt habit, but if they don't, consider another
important trial.

Cutting sodium, cutting risk

The two DASH trials evaluated diet and blood pressure; although
subsequent studies suggest this diet produces protection against
disease, DASH itself did not evaluate clinical events. But the
two larger, longer Trials of Hypertension Prevention (TOHP)
studies demonstrate that salt restriction both lowers blood
pressure and prevents disease.

The first trial (TOHP I) evaluated 744 volunteers between 1987
and 1990, and the second (TOHP II) enlisted 2,382 people between
1990 and 1995. All the participants were between the ages of 30
and 54, and all had prehypertension (diastolic blood pressure of
80–89 mm Hg). In both studies, volunteers were randomly assigned
to continue their normal diet or to reduce their sodium
consumption. In TOPH I, participants who cut their average
dietary sodium by 1,012 mg a day reduced their average systolic
blood pressure by 1.7 mm Hg and their diastolic pressure by 0.8
mm Hg. In TOHP II, a more modest sodium reduction of 920 mg a day
lowered the average systolic blood pressure by 1.2 mm Hg without
producing a significant change in diastolic blood pressure
readings.

The blood pressure reductions in TOHP I and II were so modest
that they didn't do much to advance the cause of sodium
restriction. Fast forward a decade, though, and you'll see the
power of even modest sodium restrictions and small blood pressure
reductions. When the TOHP researchers re-evaluated their subjects
10 to 15 years after the trials ended, they discovered that the
volunteers who restricted their dietary sodium during the 18 to
48 months the studies lasted enjoyed a 25% to 30% reduction in
their long-term risk for cardiovascular events.

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